1. Field of the Invention
This invention relates to a machining method using hydroxide ions in ultrapure water. More particularly, the invention relates to an electrolytic machining method and an electrolytic machining apparatus which use only ultrapure water as an electrolytic solution and increase its ion product markedly, thus being capable of removal processing a workpiece or forming an oxide film thereon by the action of hydroxide ions.
2. Description of the Related Art
In recent years, new materials have been developed one after another with the progress of science and technology. However, effective processing techniques for these new materials have not been established, and constant pursuit after forerunning new materials under development has been encouraged. Recently, microstructure and high precision have been introduced into components of every instrument. As fabrication on the submicron scale has become common, processing methods themselves have exerted increasing influence on the characteristics of materials. Under these circumstances, a processing method in which a tool effects removal processing of a workpiece while physically destroying it, such as conventional machining, causes many defects to the workpiece. As a result, the characteristics of the workpiece are deteriorated. How to process a material without impairing its characteristics poses a problem.
Among special processing methods initially developed as means for solving the problem are chemical polishing, electrolytic machining, and electrolytic polishing. These processing methods, as contrasted with conventional physical processing, carry out removal processing by causing a chemical dissolution reaction. Hence, defects due to plastic deformation, such as affected layers or dislocations, do not occur, and the above-mentioned challenge of processing a material without impairing its characteristics can be solved.
More attention has been paid to a processing method which makes use of a chemical interaction between atoms. This is a processing method utilizing fine particles or a radical with high chemical reactivity. Such a processing method performs removal processing through a chemical reaction, on the atomic order, with a workpiece. Thus, it is capable of processing control on the atomic order. Examples of this processing method are EEM (elastic emission machining) and plasma CVM (chemical vaporization machining) which were developed by the inventor of the present invention. EEM utilizes a chemical reaction between fine particles and workpiece, and realizes processing on the atomic order, without impairing the characteristics of the material. Plasma CVM utilizes a radical reaction between radicals generated in atmospheric plasma and workpiece, and realizes processing on the atomic order.
With the aforementioned electrolytic machining or electrolytic polishing, processing has hitherto been said to proceed by the electrochemical interaction between the workpiece and an electrolytic solution (an aqueous solution of NaCl, NaNO3, HF, HCl, HNO3 or NaOH). As long as the electrolytic solution is used, contamination of the workpiece with the electrolytic solution is unavoidable.
The inventor estimated that in a neutral or alkaline electrolytic solution, hydroxide ions (OH−) must take part in processing, and speculated that processing would be possible with water containing traces of hydroxide ions. Experimentally, the inventor confirmed the possibility for such processing. Based on the findings, the inventor proposed in Japanese Unexamined Patent Publication No. 58236/1998 a method which uses only ultrapure water except for traces of incidental impurities, and applies thereto a hydroxide ion increasing treatment for increasing its ion product. According to this method, a workpiece immersed in ultrapure water having an increased concentration of hydroxide ions is subjected to a chemical dissolution reaction or an oxidation reaction with hydroxide ions, whereby removal processing or oxide film formation takes place. As the hydroxide ion increasing treatment, the inventor also proposed the use of an electrochemical reaction occurring on a solid surface having an ion exchange function or a catalytic function. These proposals have led to the creation of a novel processing method capable of clean processing, with no impurities left behind on the processed surface, by utilizing hydroxide ions in ultrapure water. This processing method is expected to have wide varieties of applications, including the production of semiconductors.
However, it is a well known fact that the concentration of hydroxide ions in ultrapure water is very low and about 10−7 mol/l at 25° C. and 1 atmosphere. Even with the use of a conventional ion exchange membrane, the hydroxide ion density increased thereby is at most about 103- to 104-fold. This value is {fraction (1/10)}4 to {fraction (1/10)}3 of the ion density of 1N NaOH. It would make the processing speed still too low to realize practical processing.